Volcanic bomb is one of the most common eruption products around their source craters. Although paleomagnetic studies on volcanic bombs have a potential to provide high-resolution chronology of volcanic activity, particularly when compared with the known geomagnetic secular variation records, there are only a few such studies. In this contribution, we made an attempt to determine paleomagnetic directions from large (>1 m in diameter) volcanic bombs around “Tsubakuroswa craters”, located in Azuma volcano, for evaluating the potential use of volcanic bombs for paleomagnetic dating. Six oriented mini-cores were drilled from the central part of each large volcanic bomb, five in total, located on a gentle slope a few hundred meters south from the craters. All of the mini-cores were subjected to thermal demagnetization analysis, giving a well-determined characteristic remanent magnetization (ChRM) direction for each bomb as follows: site mean declination (Dm) of 350.6‒358.0º and inclination (Im) of 48.9‒50.8º with a 95 percent confidence limit (α95) smaller than 2.4º. The ChRM directions were consistent among the bombs, supporting the availability of volcanic bombs for further paleomagnetic dating research. Referring the geomagnetic secular variation record in this area, an all-site mean ChRM direction from the five bombs (Dec＝355.5º, Inc＝50.1º, α95＝1.9º) most likely accounts for the derivation of the volcanic bombs by the Meiji Era (1893 CE) eruption. Historic pictures and descriptions are consistent with and support this interpretation. Previous reports suggested that the Meiji Era eruption did not eject magmatic materials and that the last magmatic eruption of this volcano was probably in 1331 CE. However, our results suggest that magmatic eruptions might have occurred here only ca. 130 years ago and may be largely affecting the current activity of this crater area. Our study suggests that volcanic bombs are potentially useful materials for paleomagnetic studies such as dating and establishing geomagnetic secular variation records.
